12 Hour Clock / Infer PM when a typed schedule time has already passed

I have a minor annoyance where I try to schedule a tweet in the afternoon for a time, for example, I will write in the schedule box "today 5:00," and it will say today at 5 am has already passed. I just want to be able to schedule without always needing to write 5:00 pm. Very minor annoyance, but it would be nice to have as I am obviously not trying to schedule for the past.